3. Exposure

Exposure

The exposure is performed through masks in suitable exposure systems such as e.g. steppers (i-, g-line), mask aligners or contact exposure systems in the respective spectral working range. Direct laser exposure without masks is also possible.

AR photo coatings are light-sensitive in the broad band UV range (300 - 450 nm) and thus also at the typical emission lines of mercury at 365 nm (i-line), 405 nm (h-line), and 436 nm (g-line) (→ Absorption spectra), with maximum

sensitivity in the g- and h-line range. Values for recommended exposure dose as specified in our product information are only guideline values determined for our

standard processes and have to be confirmed accordingly in own experiments.

Air bubbles may develop either during or after exposure and are e.g. caused by too high light doses or exposure intensities. This can be avoided if the optimum light dose is determined by exposure bracketing or in several consecutive

exposure steps with intermediate pauses. A too short or too low tempering after coating results in insufficient drying of the resist film, since still too much solvent

is present in the films which causes bubble formation due to outgassing.

The exposure dose which is required to develop a large area of positive resists without structures in a suitable development time is called “dose to clear”. This exposure dose should be increased slightly for patterning, depending on the desired resolution. The maximum resolution requires the highest exposure dose.

The dose to clear unexposed areas of negative resists is in a range of 30 - 40 s for films with a thickness of 1 - 2 μm. This exposure dose which produces a layer buildup of > 90 % should accordingly be increased by 10 - 20 % for patterning

undiluted poses. For thick films of more than 100 μm, development times of more than 1 hour may be required.

Coated and tempered resist films can be stored for several weeks prior to exposure without quality loss. Photoresists are however more sensitive directly after coating as compared to layers which were stored for several hours or days. The decrease in sensitivity is approximately 3 % after 3 h, 6 % after 72 h, and 8 % after 72 hours (in relation to the initial value) and remains then more or less constant for several weeks.